Institute of Geophysics and Space Physics and
Department of Earth and Space Sciences
University of California, Los Angeles
California 90024-1567
January 1991

in Science Progress, 75, 93-105, 1991

Abstract

The study of magnetospheres is of great scientific interest and practical
importance. Magnetospheres provide ideal laboratories in which to study the
behavior of plasmas, a state of matter being increasing important in our quest
for new energy sources. Moreover, the Earth's magnetosphere is host to numerous
expensive satellite systems, and often not a benign host. All planets visited
to date have magnetospheres whether the planets have an internally generated
magnetic field or not. This article provides a brief overview of these
planetary magnetospheres and compares several processes as they occur at
different planets.

Introduction

All planets and comets explored to date have magnetospheres. The existence
of these magnetospheres is independent of whether the planet has an internally
generated or intrinsic magnetic field, but the nature of these magnetospheres is
quite dependent on this fact. For the planets that have no internal magnetic
dynamo the solar wind induces a magnetosphere through its interaction with the
upper atmosphere and ionosphere. We will distinguish between these two types of
magnetospheres by calling them intrinsic and induced magnetospheres according to
the source of their magnetic fields.

The study of planetary magnetospheres may at first seem arcane and exotic,
and perhaps of little practical importance. The physical processes that take
place in magnetospheres involve rarefied gases, often nearly completely ionized,
in which collisions seldom occur. Despite the absence of collisions these
ionized gases, or plasmas as they are usually called, behave much like
collisional gases or fluids, with coherent behavior induced by their electric and
magnetic fields. How the analogues of ordinary processes such as diffusion and
dissipation take place in these systems are of immense interest to space plasma
physicists. Also of great interest are the various phenomena such as magnetic
reconnection and Landau damping that have no analogues in ordinary gases. These
processes are also of importance to astrophysicists and plasma fusion physicists.
In the former case, the planetary magnetospheres provide in-situ data for
processes that may occur on a grander scale elsewhere. In the latter case space
provides plasmas without wall effects and often with more complete plasma
diagnostics.

In addition to these academic motivations there are two very practical
reasons to study planetary magnetospheres. First, the Earth has an internally
generated magnetic field whose source we still do not understand. We would hope
by studying the generation of magnetic fields at the other planets we might
better understand our own. In order to determine the characteristics of these
fields we must first understand, especially for the weakly magnetized planets,
the nature of the external contributions to the magnetic field, those of the
planetary magnetosphere. Secondly, we live in an increasingly technological
society on a planet with a significant magnetosphere into which we continue to
launch sophisticated spacecraft critical to that technological society. The
operations of these spacecraft are affected by that environment. In fact several
expensive spacecraft have ceased operations because of large magnetospheric
disturbances.

The effects of these magnetospheric disturbances are not restricted to the
regions well above the surface of the Earth. Power blackouts have been caused
by the intense voltage surges induced in long distance power distribution
systems. Communication disruptions have been produced as the effects of these
disturbances alter the properties of the ionosphere. Finally, these disturbances
cause immense auroral displays. At usual times these displays are seen mainly
over Siberia, Alaska, Northern Canada and Scandanavia and their southern
counterparts but at disturbed times have been seen as close to the equator as
Mexico and Japan.
It could be asked why we should study these phenomena at planets other than the Earth.
The data obtainable from terrestrial satellites must be of higher quality and
quantity than those returned by flyby missions to the distant reaches of the
solar system. We need to travel abroad in the solar system because the
terrestrial magnetosphere presents us with a limited range of boundary
conditions and scale sizes with which to test our theories. The solar wind varies greatly
with radial distance from the sun and the external conditions it imposes on the
various magnetospheres likewise change. These changes and the varying strength
of the magnetic fields of the planets also cause the sizes of the magnetospheres
to vary greatly. The largest magnetosphere easily could contain the sun and its
corona; the smallest could be contained inside the volume of the Earth.

In this review we outline the basic physical processes that occur in both
intrinsic and induced magnetospheres, review briefly the highlights of our
exploration of these planets and what problems remain, and show some examples of
how the same physical process varies as it occurs in different regions of the
solar system.

Induced Magnetospheres

The sun emits a constant stream of electrons and protons in all directions
at speeds well above the speed of "sound". This supersonic ionized gas, or
plasma, called the solar wind carries with it a magnetic field and a frame
dependent electric field. The frame-dependence arises due to the high
electrical conductivity of the plasma and its magnetic field.
In the frame moving with the plasma the electric field under most
circumstances is zero. There is no electric field parallel or perpendicular
to the magnetic field. In a frame not moving with the plasma there is an
electric field perpendicular to the magnetic field and to the velocity
vector proportional to both the magnetic field and the component of velocity
perpendicular to the magnetic field. This electric field is very important
for the removal of a planetary atmosphere from an unmagneticed planet.

Solar extreme ultraviolet radiation ionizes the upper atmospheres of all
planets to varying degrees. If the thermal pressure of this ionosphere exceeds
the solar wind momentum flux or dynamic pressure, a quantity proportional to the
density times the square of the velocity, then the ionosphere can stand off the
solar wind and it remains unmagnetized. A magnetic lid or cap forms on the
ionosphere called the magnetic barrier and this barrier in turn deflects the
solar wind. The solar wind as mentioned above is supersonic and thus this
deflection must involve the formation of a detached bow shock. This bow shock,
which interestingly forms without the aid of collisions in the gas, slows, heats
and deflects the solar wind.
Figure 1 shows a cross section of this
interaction.

Fig.1. Schematic of the solar wind interaction with an unmagnetized planet.
Horizontal lines which curve around the planet represent streamlines of the
solar wind flow. Flow proceeds from left to right. Vertical lines represent
the interplanetary magnetic field which is carried to the planet by the solar
wind and draped over it.

The behavior of the ionosphere in such an interaction is quite unexpected.
Although the thermal pressure of the ionosphere may be strong enough to hold off
the solar wind, still small magnetic filaments or magnetic flux ropes sink from
the magnetic barrier into the ionosphere, providing an opportunity to study, in-
situ, a phenomenon otherwise seen only remotely on the solar surface. When the
solar wind dynamic pressure is high and exceeds that of the thermal ionosphere
magnetic field and plasma is pushed downward into the ionosphere and it acquires
a steady global magnetic field.

The induced magnetosphere has one more very important feature. The solar
wind moves past the planet at supersonic speed carrying its magnetic field with
it. Near the planet the flow is slowed. The magnetic field that connects the
fast and slow regions must perforce be distorted as shown in
Figure 2 leading to
the generation of a magnetic tail. The interaction can pick up mass from the
ionosphere, and through ionization from the atmosphere. This further slows the
flow near the planet and increases the magnetic flux in the tail. The bend in
the magnetic field and gradients in field strength act to accelerate the plasma
in the antisolar direction. Much plasma can reach escape velocities by this
mechanism.

Another route for atmospheric loss is the electric field of the solar wind.
If particles are ionized in the magnetized flow, they will be quickly accelerated
by the electric field and if the direction of the acceleration is correct they
can spiral out into the solar wind as illustrated in
Figure 3. The combined
effect of the electric and magnetic fields of the solar wind acts to remove
atmospheric gases from the unmagnetized planets only some of which is replenished
by the absorption of the incoming solar wind.

Venus
The magnetic moment of Venus is less than one hundred thousandths of that
of the Earth and plays no role in the solar wind interaction with the planet.
Venus has been extensively explored in the Soviet and American programs with the
Mariner 2, 5 and 10 flyby missions, the Venera 2, 4, 6, 8-14 landers;the Venera
9 and 10 orbiters and the Pioneer Venus atmospheric probes and orbiter. The
orbiter missions especially have revealed much of the understanding outlined
above. Nevertheless we still do not know how much atmosphere is being lost to
the solar wind, nor do we understand many of the phenomena found to occur in the
ionosphere such as the formation of magnetic flux ropes.

Mars
The precise size of the magnetic field of Mars is not known but its
strength is probably much less than one ten thousandths of that of the Earth and
like Venus the intrinsic magnetic field is not significant for the solar wind
interaction. The Martian magnetosphere has been studied by the Mariner 4 flyby
mission and the Mars 2, 3, 5 and Phobos orbiters. The ionosphere is thought to
be magnetized because the solar wind dynamic pressure exceeds the thermal
pressure of the ionosphere but no measurements have been made to confirm this
hypothesis. Other features, such as the bow shock and magnetotail, are very
similar to those of Venus. We have better measurements of the loss of the
Martian ionosphere due to the solar wind interaction taken on the Phobos mission
but at this writing these data are not yet fully reduced.

Comets
Comets are much smaller objects than planets if only their nuclei are
considered. Their much smaller mass means that gravity is not a factor in the
solar wind interaction. The size over which the cometary gas can spread in the
solar wind is thus controlled by the speed of expansion of the cometary gas
(about one km/s) and the ionization time (about a day at 1 AU from the Sun).
Their product is about 105 km which is much larger than the size of the
interaction regions at Venus and Mars. Not only does the interaction cover
greater territory but it is much more gradual. Thus, for example, the bow shock
is much weaker at a comet because much of the ionization forms ahead of the
region where the bow shock forms so the solar wind is slowed prior to the shock.
Measurements at and near comet Halley were made by five spacecraft Vega 1 and 2,
Giotto, Sakigake and Suisei. Measurements from the smaller comet Giacobini-
Zinner, were obtained from the ISEE-3 spacecraft. In no case were measurements
made in the fully developed cometary tail. The data returned by these missions
provided interesting insights into the physics of cometary magnetospheres but
mainly whetted the appetites of cometary physicists. A mission that matches
trajectories with a comet and can take long-term measurements is needed before
the processes occurring at a comet are fully understood.

Intrinsic Magnetospheres

For the magnetized planets, those with intrinsic magnetic fields, the
obstacle to the solar wind is the planetary magnetic field and the size of the
magnetosphere is governed by the relative strengths of the magnetic field and the
solar wind at the planet. The strength of a planetary magnetic field is given
by its dipole magnetic moment, the equatorial surface field strength times the
cube of the planetary radius. The dipole magnetic field falls off as the cube
of the radius of the planet. Since the pressure balance is established between
the magnetic pressure and the solar wind dynamic pressure at the subsolar point
and since magnetic pressure is proportional to the square of the magnetic field
strength, the sizes of planetary magnetospheres are proportional to the sixth
root of the dynamic pressure. Table 1 lists the dipole magnetic moments for all
of the planets, the average solar wind dynamic pressure for each planet which
decreases as the square of the distance from the sun and the expected location
of the pressure balance point along the subsolar direction. Only one planet,
Jupiter, fails to follow this simple relation. At Jupiter part of the outward
pressure is supplied by rapidly rotating plasma supplied by the volcanoes of Io.
As the Table shows the magnetosphere of Mercury is clearly the smallest and that
of Jupiter is by far the largest.

The magnetosphere of the Earth is of course the magnetosphere that has been
most thoroughly studied. Because the properties of this magnetosphere generally
lie in the middle of the range of properties found in the solar system we can
regard the terrestrial magnetosphere as typical.
Figure 4 shows a cut away
drawing of the magnetosphere. The outer boundary of the magnetosphere is called
the magnetopause, upon which flows the magnetopause current, a large current
vortex which separates the magnetic field of the Earth and the solar wind.
Behind the Earth are the two lobes of the magnetic tail, the top one pointing to
the Earth and the bottom one pointing away. These magnetic field lines enter and
leave the Earth in oval shaped regions known as the polar caps. These polar caps
vary in size as solar wind conditions vary. This variation plays a very
important role in energy transfer into the magnetosphere and will be discussed
in greater detail below. Between the two tail lobes flows the neutral sheet
current which is simply part of the magnetopause current vortex and also the
plasma sheet a hotter and denser plasma than in the surrounding regions. The
production of this plasma sheet is one of the areas of most intense study at the
present time.

Deeper in the magnetosphere we find the plasmasphere, a region of dense
cold plasma which is the upper extension of the ionosphere. The plasmasphere
extends out to about 5 Earth radii. Within this distance magnetic flux tubes
fill up with cold plasma from the ionosphere below. Outside this distance the
filling time is long compared to the transport and loss time so the magnetic flux
tubes do not fill up with cold plasma.

The closed, dipolar field lines in the magnetosphere provide efficient
magnetic mirrors in which to trap energetic particles. Close to the Earth these
radiation belts are very stable and can remain constant for hundreds of years but
in the outer regions the belts are subject to frequent disturbances and change
from day-to-day. Particles from the outer regions can cross the field lines by
diffusion and convection. Diffusion is a slow process which relies on
fluctuations of the magnetic and electric fields. Convection refers to the
drifts induced by the large scale electric field in the magnetosphere. It is
important only for low energy particles and only in the outer parts of the
magnetosphere.

If one pushes or pulls on the outer parts of the magnetosphere, one would
expect the stresses created by that action to affect the plasma in the Earth's
ionosphere for the ionosphere is where the magnetosphere is coupled to the Earth.
The magnetosphere communicates this stress through field-aligned currents.
Figure 4 shows the paths of some of these currents.

Mercury
The magnetic moment of Mercury is about one 1/3000th of the terrestrial
magnetic moment. The equatorial surface magnetic field strength is about 250 nT.
Mercury has been explored by only one spacecraft Mariner 10 which passed by
Mercury 3 times in 1974 and 1975. On two of these passes the spacecraft passed
through the wake of the planet encountering a mini-magnetosphere much like that
of the Earth. These two passes gave us only a brief glimpse of the nature of the
Mercury magnetosphere. This glimpse was not enough to precisely determine the
strength of the magnetic moment of the planet. It did however suggest that the
magnetosphere more efficiently extracts energy from the solar wind than does the
Earth's magnetosphere. Scientists hope to revisit Mercury in the future with
one or more orbiting spacecraft, but presently it is expected that this will not
happen until early in the 21st century.

Earth
The equatorial surface field of the Earth is about 31,000 nT. It is strong
enough to activate rudimentary magnetic compasses and has been used as a
navigational aid for at least 1000 years. The investigation of the earth's
magnetic field began in about the 16th century but reached its zenith in the
space age when it could be more fully explored with spacecraft. The spacecraft
which have examined the Earth's magnetosphere are too numerous to name and have
been launched by all the spacefaring nations. At present the most active area
of research in magnetospheric physics is energy transfer from the solar wind to
the magnetosphere. In the mid-1990's a consortium of space agencies (ESA,
Intercosmos and NASA) are going to launch a flotilla of spacecraft into the
magnetosphere to study this problem. This program is called the International
Solar Terrestrial Program and will consist of over 15 different spacecraft.

Jupiter
The magnetic moment of Jupiter, as befitting the largest planet in the
solar system, is also the largest of the planetary system over 10,000 times that
of the earth. Its equatorial surface field is over 10 times that of the Earth.
The strength of its magnetic field combined with the weakness of the solar wind
at Jupiter produces a magnetosphere that is enormous. The sun could easily fit
inside the magnetosphere. Its tail is thought to extend past Saturn, over 5 AU
away. If Jupiter's magnetosphere could be seen from Earth it would appear to be
larger than the Earth's moon.

Deep inside the jovian magnetosphere orbit the Galilean satellites. One
of these, Io, has a volcanically produced atmosphere that is constantly being
bombarded by the intense radiation belts of jupiter. This bombardment knocks
atoms out of the atmosphere of Io into the magnetosphere of Jupiter where they
become ionized. This process produces a torus, or doughnut, of hot ions circling
Jupiter near Io's orbit. This torus together with the enormous electrical and
magnetic forces in the Jovian magnetosphere leads to intense radiation belts and
radio emissions. These emissions can be detected from Earth and were the first
indication of Jupiter's enormous magnetic field well before the first
interplanetary spacecraft were launched.

Jupiter has been visited four times by spacecraft: Pioneer 10 in 1973;
Pioneer 11 in 1974; and Voyager 1 and 2 in 1979. Each of these spacecraft were
on flyby trajectories. At this writing the Galileo spacecraft is on its way to
Jupiter when it will be injected into an elliptic near equatorial orbit in 1995.

Saturn
The magnetosphere of Saturn is quite benign compared to that of Jupiter.
Since Saturn is a smaller planet, its conducting core in which the planetary
magnetic field is generated is smaller, and so is the planetary magnetic field.
The magnetic moment of Saturn is 580 times that of the Earth but its equatorial
surface magnetic field strength is about equal that of the Earth. In stark
contrast to the magnetic fields of all the other planets, the Saturnian dipole
moment is not tilted with respect to the rotation axis of the planet. This
observation was a great surprise to those studying planetary magnetic dynamos.
Saturn's ring system absorbs radiation belt particles so that the radiation belts
are weaker than at Jupiter and none of Saturn's moons exhibits volcanic activity
similar to that of Io. As a consequence Saturn's radiation belt resemble more
those of the Earth than those of Jupiter and few radio emissions are produced.

Saturn has been visited by 3 spacecraft Pioneer 11 in 1979, Voyager 1 in
1980 and Voyager 2 in 1981. Each of these were on flyby trajectories.
Currently, NASA and ESA are working on an orbiter/probe mission called
Cassini/Huygens which is scheduled to arrive at Saturn early in the 21st century.

Uranus and Neptune
The magnetic fields of Uranus and Neptune are quite unlike those of the
other planets. The magnetic fields are quite irregular and cannot be well
represented by a simple dipole field. When a dipole moment is fit to the flyby
data available from Voyager 2 which flew by these planets in 1986 and 1989
respectively, a very large tilt angle between the rotation axis and the dipole
axis is found, about 50o. The magnetic fields are also much weaker than those
found at Jupiter and Saturn. The magnetic moments are about 40 times that of
Earth and their surface magnetic fields slightly less than the terrestrial field.
The reason for this weakness and the irregularity may be that the magnetic field
is generated, not in a deep molten core like the Earth's, but in salty ice/water
oceans closer to the surface. The radiation both of Uranus and Neptune are quite
weak. There are no present plans to explore these planets further.

Comparative Magnetospheres

The magnetospheres of the planets differ both in size and internal energy
sources but also in the strength of the solar wind flow past their surfaces.
Thus, the interaction of each of the magnetospheres with the solar wind differs
in some degree from the others. Herein we examine how some of these processes
vary from planet to planet.

The Bow Shock
The bow shock is a standing wave in front of a magnetosphere at which the
supersonic solar wind is slowed, heated, and deflected around the planet. The
strength of this shock depends on the flow velocity of the solar wind relative
to the velocity of compressional waves in the plasma. This latter velocity
decreases with increasing distance from the sun while the former remains quite
constant. As a result, the strength or Mach number of the bow shock increases
markedly from the inner solar system to the outer solar system. At Mercury the
bow shock has a Mach number of about 4 but at Neptune it is about 20. At low
Mach numbers the shock is found to be quite smoothly varying or laminar in
appearance but at high Mach number the shock becomes very turbulent.

Upstream Waves
The bow shock represents an obstacle to some of the solar wind particles
and they are reflected back upstream along the magnetic field. These
counterstreaming particles cause waves to grow in the solar wind. These waves
cannot propagate upstream against the solar wind and are blown back against the
planetary shocks. The number of particles reflected by a planetary bow shock
increases with the strength of the bow shock. Thus the strength and
characteristics of the upstream waves change with heliocentric distance.
Figure 5 shows one such property of the waves, their frequency. As one moves outward
in the solar system the frequency of the waves change in proportion to the field
strength as would be expected if the waves were associated with a gyro resonance
with the reflected solar wind ions.

Reconnection
Another process that appears to be influenced by the Mach number is the
phenomenon known as reconnection. In this process magnetic field lines in the
solar wind link up with those of the planetary magnetosphere, thereby increasing
the tangential stress on the magnetosphere and adding magnetic energy to the
magnetotail. Under solar wind conditions typical of those in the inner solar
system this process is controlled principally by the direction of the solar wind
magnetic field relative to the direction of the planetary magnetic field. When
these directions are antiparallel, reconnection takes place readily and, when
they are parallel, it does not take place at all. However, when solar wind
conditions change to those typical of the outer solar system reconnection seems
to cease. This is illustrated in
Figure 6 which shows the reconnection
efficiency judged from terrestrial records of geomagnetic activity versus solar
wind Mach number. It shows that about a Mach number of 7 the reconnection rate
appears to go to zero. Thus reconnection is expected to be more important in the
inner solar system where the Mach number is typically 7 or less than in the outer
solar system where it is often 10 or greater.

An associated phenomenon is that known as the Flux Transfer Event which
appears to be the signature of temporally and spatially varying reconnection.
These features have been observed at the magnetopauses of Mercury, Earth, and
Jupiter. At Mercury these events are of short duration, about 1 s and occur
frequently about every 30 s. At Earth these features last about 30 s and occur
about every 5 minutes. At Jupiter the signature is similar to that at the Earth.
This observation suggests that the small size of the Mercury magnetosphere
affects the generation of Flux Transfer Events. However, at Earth and Jupiter
the size of Flux Transfer Events may be controlled by some other property of the
magnetosphere such as the thickness of the magnetopause which is the same at both
planets.

Summary

In the sections above we have outlined the general features of planetary
magnetospheres. Some of these magnetospheres are induced and some intrinsic.
Both types stand off the solar wind flow and cause planetary bow shocks. The
variation of the solar wind with distance from the shock together with other
planets to planet differences causes a spectrum of responses to the solar wind
flow. These differences in turn allow us better to understand the processes
taking place. The space missions to these planets over the last 2 decades have
returned a wealth of data about their magnetospheres, data through which we are
still sorting. Many mysteries have been answered with the acquisition of these
data, yet many mysteries remain. Thus we look forward to the upcoming missions
such as the International Solar Terrestrial Program, Galileo, Cassini and Mercury
Orbiter to help solve these problems.
Acknowledgments

The preparation of this report was supported by the National Aeronautics
and Space Administration under research grant NAS2-501.